Such elements, typically cylindrical in form and of lengths of the order of 1000 or 1250 mm, commonly also known as “sections”, are used for heat insulation of heat-transfer fluid ducts. They consist of a punched rectangular sheet-metal blank, which is then roll-bent, remaining open along a generating line, so as to be capable of lateral insertion around the duct to be heat-insulated. The two opposing longitudinal edges are then joined together, by means of machine bolts for example. Furthermore, the axial ends of these heat-insulating elements are shaped such that the elements can be fitted together by push-fitting. The general principle used for fitting together in a straight line is push-fitting using male and female moldings, this push-fit being capable of ensuring good water-tightness and sufficient rigidity, be it for installation in a vertical or horizontal position.
Push-fitting of heat-insulating elements may however be implemented using various techniques, some examples of which are illustrated in FIGS. 1 to 4, which show fitted-together pipe elements, each pipe 1 consisting of a rolled blank of sheet metal closed by joining of the longitudinal edges 18, 19. FIG. 1 depicts a push-fit with an embossed peripheral molding 11 towards one smaller diameter end 12 of the pipe 1, the molding constituting a stop when said smaller diameter end is push-fitted inside the opposite cylindrical end 13 of another pipe. FIG. 2 shows a push-fit system which is similar but with necking 14 instead of the “conical shape” of the previous example, this necking being obtained by the formation of axial corrugations distributed over the periphery of the pipe. FIG. 3 depicts a push-fit of the edge-to-edge type, one end comprising a recessed molding 15 formed right at the edge of the pipe, into which is inserted the edge of an embossed molding 16 formed on the other end. FIG. 4 depicts a push-fit of the overlapping type, with one end comprising necking 14 and corrugations and a recessed molding 15 at the boundary of the necking, and the other end of the pipe being inserted over said necking and additionally comprising an embossed molding 16, the edge of which is inserted into said recessed molding.
In every case the moldings or corrugations formed towards the axial ends of the elements create deformation of the entire thickness of the sheet, which has furthermore to remain smooth and without deformation between these deformed end zones.
The material of the sheets used to manufacture these pipes largely depends on the application. They may in particular be of aluminum, prepainted steel, stainless steel or electroplated steel, the thickness of the sheet generally varying between 0.4 and 1 mm.
Currently, manufacture of heat-insulating elements from sheets supplied in coils comprises the following series of steps:                A) cutting the blank to size (width 1000-1250 mm, length=diameter*π+overlength for overlap),        B) punching the blank to form holes for bolts to close the pipe,        C) longitudinal molding to form one or two moldings also used for closure purposes,        D) rolling, or roll-bending, the sidewall to the diameter required to obtain a shell, using a rolling machine,        E) preassembling the shell with bolts to achieve sufficient rigidity,        F) shaping the ends, for example with the shapes described above, to allow the pipes to be fitted together end-to-end. Shaping of the ends is performed using an edge-forming machine.        G) removing the bolts to allow the shells to be fitted one inside the other, in particular to reduce bulk during transport.        
The operation of shaping the ends is very important and must be performed very carefully by a professional, the depth of the molding and the repeatability of the result being important so as subsequently to ensure uniform assembly and fitting together of the heat-insulating elements.
Edge-forming machines are known, in particular from EP1518616, EP1724033, EP1213063, which are dedicated to this shaping. However, overall, molding shaping operations for the most part remain manual and involve a multitude of operations, causing operators physical fatigue. Likewise, the repeatability of the operations on the edge-forming machine is dependent on the operator, the accuracy of the molding depth being a function in particular of the operator's dexterity.
Furthermore, rolling and molding operations, in which a male side and a female side are produced, bring about deformation of the cylindrical element, one side being under-rolled and the other over-rolled, the heat-insulating elements ultimately being deformed.
These problems are increased further if sheets are used which are difficult to work, such as thick sheets, for example of grade 316 stainless steel up to a thickness of 1 mm, which will require more effort than a thin sheet of aluminum. The loading exerted for the rolling operation and above all the molding operation must be sufficient to achieve the desired deformation. It may consequently be necessary to perform several passes on the edge-forming machine, modifying the settings of the edge-forming machine between each pass to arrive at the desired degree of relief or molding depth.
A further drawback is that there is risk of the surface of the shell or pipe being damaged during the various operations required and in particular in the case of several successive passes in the edge-forming machine. In the case of coated steel blanks, such damage poses an aesthetic problem but also a risk of subsequent corrosion.
Still another drawback is that this method of manufacture requires pre-assembly, with bolting, before end-shaping is performed, and also requires, after shaping, removal of the bolts used for pre-assembly of the edges and disassembly of said edges to package the shells, fitted one inside the other, for transport. Apart from the time loss resulting from these operations, they increase the risk of damage to the surface of the heat-insulating elements.
U.S. Pat. Nos. 3,858,785 and 3,921,883 additionally disclose methods for manufacturing tubes comprising peripheral reliefs, or corrugations, where said reliefs are formed on the sheets prior to the latter being roll-bent to form a tube. However, these tubes are welded tubes, intended for heat exchangers, and either the reliefs are formed on one or two faces but in such a way that the relief on one face has no effect on the other face, as in U.S. Pat. No. 3,858,785, or the corrugations are produced continuously over the entire length of the tube, as in U.S. Pat. No. 3,921,883. Apart from the fact that these tubes, intended for heat exchangers, do not have at all the same features and functionalities as the heat-insulating tubes according to the invention, the reliefs imparted to them do not pose at all the same problems as the heat-insulating elements to which the invention relates, and in particular the manufacturing methods of U.S. Pat. Nos. 3,858,785 and 3,921,883 do not allow the production of tubular elements of which only the ends are shaped to obtain peripheral moldings or corrugations while the part located between these ends remains uniformly smooth without deformation other than that resulting from roll-bending of the sheet to make it tubular.